It has nothing to do with QC of the shoes, everything to do with the mould they use to make their shoes. Different companies have different foot "shapes" they base their shoes/boots off. For example, taking ski boots as an example but the same system applies for all shoes/boots:
Two major brands of ski boots, Salomon and Technica. Salomon foot shapes/moulds are longer, narrower and generally lower volume than Technica, which are generally the widest and highest volume on the market. Basically for the same sized boot the fit and shape are totally different, one is much wider and higher than the other.
It's why you should try boots/shoes on before buying if you're getting them from a different brand. The alternative is to pretty much just stick with the brand you wear when buying online, as you know they fit and you know what size you are. I can be anything from a size 9-11 depending on which brand I buy, all depends on the shape of their moulds.